Tractors are commonly fitted with a three-point hitch to allow an implement, such as a plough or a harrow, to be connected to them. A three-point hitch comprises three movable arms, namely two lower lifting arms and an upper arm, that is also termed a top link, positioned centrally in relation to the lifting arms. The two lifting arms are raised and lowered by hydraulic cylinders that allow the towed implement to be raised, lowered or tilted. The top link is also movable but it is not usually hydraulically powered.
In a lowered position, the lifting arms should be allowed to move laterally from side to side. However, in a raised position, that is used for example when towing an implement on metalled roads, the lifting arms should be prevented from moving laterally in order to prevent yawing, or fish-tailing, of the implement.
To meet these needs, a stabilising mechanism is connected to the lifting arms, an example of a stabilising mechanism being shown in FIG. 1 of U.S. Pat. No. 5,462,303.
The stabilising mechanism includes a sprung telescopically collapsible strut, connected between one of the lifting arms and the tractor chassis. In the lowered position of the lifting arms, the two ends of the strut can move relative to one another against the action of the springs, so that the lifting arms are biased by the springs of the strut into the central position but are still capable of moving from side to side.
A lockable strut is connected between the other lifting arm and the tractor chassis. Though a separate strut is described in U.S. Pat. No. 5,462,303, the sprung telescopically collapsible strut may itself be lockable. The lockable strut is formed of two members that can slide, for example telescopically, relative to one another when the lifting arms are lowered but are prevented by a locking pin from moving relative to one another when the lifting arms are raised so as to lock the lifting arms of the hitch in a central position, symmetrical about the longitudinal axis of the tractor.
The locking pin of the lockable strut is biased towards its locked position and is attached by a flexible linkage, hereinafter often referred to as a chain though it may alternatively be a cable, to a point on the tractor chassis. The length of the chain is selected such that, when the lifting arms are lowered, the chain becomes taut and automatically retracts the locking pin to allow free lateral movement of the lifting arms.
In order to retract the locking pin smoothly and without risk of damage, it is desirable for the tension of the chain to act along the axis of movement of the locking pin, which is generally substantially vertical. However, as can be seen from FIG. 1 of U.S. Pat. No. 5,462,303, there is usually no convenient fixing point on the tractor chassis that is aligned with the axis of the locking pin and a compromise position needs to be adopted for anchoring the opposite end of the chain, the position being selected to be as close as possible to the line of action of the locking pin.